Heating system

ABSTRACT

A heating system for a building comprises a boiler and radiators to which hot water is supplied from the boiler, in which at least some of the return water flow from the radiators, typically at 6° C., circulates through a first heat exchanger to cool the exhausting products of combustion, means is provided to direct a small part of the return water flow through at least one heat exchanger which gives up heat to the building and/or to the boiler heating the water for heating he building, to cool the diverted water to well below the dew point, and means is provided to supply the cooled water to a second heat exchanger through which the already cooled exhaust products of combustion have to pass after leaving the first heat exchanger, to achieve full condensation of the water vapour content of the products of combustion form the boiler.

FIELD OF INVENTION

This invention concerns condensing boiler based heating systems.

BACKGROUND TO THE INVENTION

A condensing boiler system will cause the steam of combustion of a fuelto condense to liquid water and will also collect the latent heat ofvaporisation of the steam and recycle this heat into the boiler systemand thus increase the thermal efficiency of the boiler system.

Thus it has previously been proposed to recover heat from the flue gasesof a water boiler in earlier UK Patent Application No. 0107963.1, usinga secondary heat exchanger, which raised the gross efficiency of aboiler from about 89.5% to about 92.5%.

The following points have been noted in relation to water boilers:

-   1. The flue gas temperature from a boiler without a condensing    secondary exchanger is well above 100° C., typically in the range    150°–250° C., and it is quite certain that such a boiler recovers no    latent heat from steam because the flue gas temperature is too high    for any condensation to take place. If then, such a boiler operates    with a gross efficiency of nearly 90%, then it is possible to    conclude that all the steam latent heat is still trapped in the flue    gases effluent from the boiler, and will escape up the flue.-   2. The same boiler, fitted with a stainless steel (or other suitable    metal) secondary heat exchanger as previously proposed, will    demonstrably operate with a gross efficiency of about 93%, an    efficiency gain of just 3%. This secondary exchanger, although    fitted with a water condensate overflow tube, has been found to    provide only a liter or so of water over an operating time of    several hours. Therefore the secondary exchanger (condenser)    described, although condensing some water and therefore correctly    termed a condensing heat exchanger, is only condensing a small    proportion of the steam, and a proper description would be a    partially condensing heat exchanger because in no way is the volume    of water collected representative of the volume of steam generated    by the burning fuel, but is only a fraction thereof.-   3. As confirmation of (2) above, large quantities of steam can be    seen issuing to atmosphere from the secondary exchanger previously    proposed, and this shows that at any rate some steam escapes the    secondary exchanger and latent heat escapes with the steam.-   4. In a typical example of a kerosene based fuel, the percentage of    hydrogen in the fuel by weight is about 13.74%. Thus it can be shown    that burning 1 US gallon per hour of the fuel will produce 3.6    kilograms of water as steam per hour. Moreover, the specific latent    heat of steam at 100° C. is 2.26 megajoules per kilogram, or almost    6% of the total calorific value of the fuel, and so far only a    fraction of this has been recovered by secondary heat exchangers as    proposed. For total latent heat recovery to take place, it follows    that about 3.6 kilograms of water could (and should) be recovered.    In practice perhaps as little as 10% is actually recovered.

Therefore although a 3% efficiency gain has been achieved, the principalamount of energy recovered by the previously proposed secondary heatexchanger is due to reducing the temperature of boiler exhaust gases.The bulk of the steam and its energy escapes to atmosphere, as evidencedby the small volume of condensed water collected

-   5. The dew point of the water vapour component in the flue gas of a    typical domestic boiler is about 50°–55° C. It is quite impossible    to fully condense this water and to capture the energy of    condensation unless the medium used to cool the gases is below, and    ideally considerably below, the dew point. The temperature of the    return water from the radiators previously proposed to cool the flue    gases is at about 60° C., and therefore is unavailable to cool the    flue gases below the dew point.-   6. It is not considered sensible to reduce the water flow through    the radiators so that the return temperature is sufficiently low    (e.g. 25°–30° C.) as to allow this water to be employed in the heat    exchanger to effect full condensation of the water vapour in the    still hot gases.

OBJECT OF THE INVENTION

It is an object of the present invention to provide an improvedcondensing boiler based heating system, capable of recovering a greaterpercentage of the latent heat in the steam present in the flue gases,and an improved condensing heat exchanger for such a boiler.

In the case of combustion of north sea gas, the latent heat in the steamcontent of the exhaust gases can account for approximately 10% of totalcombustion energy.

In the case of liquified gases such as butane and propane, the latentheat in the steam content of the exhaust gases can account for about 8%of total combustion energy.

In the case of lighter fuel oils, such as Kerosene and Gas Oil, thelatent heat in the steam content of the exhaust gases can account forabout 6% of total combustion energy.

These figures are based on typical content of hydrogen of the fuels. Thelatent heat can be taken to be proportional to the hydrogen content ofthe original fuel since the latent heat in the exhaust gases will beequal to the heat required to vaporise the water which results when thehydrogen component is burnt in air.

SUMMARY OF THE INVENTION

The invention lies in a method of cooling hot flue gases from a waterboiler in which the hot flue gases leaving the boiler are cooled byrelatively cold water at below the dew point of the water vapour contentin the flue gases, so as inter alia to give up the latent heat fromwater vapour contained in the flue gases, by reducing the temperature ofthe gases below the dew point of the water content, and in so doingheating the cold water to a higher temperature, recovering the heat fromthe heated water to assist in heating the interior of a building heatedby hot water from the boiler and in turn cooling the water again, toenable it to be re-circulated to cool the flue gases and to continue torecover latent heat from water vapour present therein.

The condensing heat exchange step may be achieved by circulating thecold water whose temperature is below the water vapour dew point,through a heat exchange device over or through which the flue gases areforced to pass.

Alternatively and preferably the flue gases may be in direct contactwith the cold water in the condensing heat exchange step, so that thecondensed water combines with the cold water and is carried away in thegeneral flow of water.

Where the condensed water vapour is absorbed into the circulating coldwater, the volume of circulating cold water will increase with time. Ina closed system, provision must be made to draw off any excess water ifthe volume of circulating water is to remain substantially constant.Typically this is achieved using a syphon or weir at a suitable point inthe cold water circuit.

There are several ways of achieving the desired results with thegas/water cooling system, which include the use of a large heatexchanger (due to the need to present a large area of water to the hotflue gases, to effect cooling of the latter) but preferably two separateheat exchangers are employed successively to cool the flue gases.

Preferably a water pump is employed to facilitate water circulationthrough the two heat exchangers and where the water circulating throughthe two heat exchangers is separate two pumps may be employed.

In any heat exchange system, the area over which heat exchange is tooccur is critical and the heat exchange surface in general will beproportional in area to the heat output from the boiler. In practice,depending on boiler capacity, heat exchanger areas are usuallydetermined by experiment. This is especially the case when consideringthe area of water to be presented to hot flue gases for direct contacttherewith. The heat exchange efficiency has been found to depend interalia on water and gas flow rates, the angle of gas flow direction to thewater surface, and also to the temperature difference between the gasesand the water.

According to a preferred feature of the method proposed by the inventionfor recovering latent heat from water vapour in the flue gases from ahydrocarbon fuel-burning boiler adapted to heat the interior of abuilding by heating water to be circulated through radiators in thebuilding, hot flue gases from the boiler are cooled first by a heatexchanger through which water returning from the radiators is caused toflow and then by a second heat exchanger through which water at atemperature below the dew point of the water vapour in the flue gases iscaused to flow.

Preferably water from the second heat exchanger containing heatrecovered from the flue gases by the second heat exchanger, is conveyedto the boiler and/or to the building interior.

Preferably the water flowing through the second heat exchanger isseparate from the water flowing through the main boiler and radiators.

The temperature of the relatively cold water in the second heatexchanger is typically in the range 20°–40° C. preferably 25°–30° C.

Preferably the water which is to flow through the second heat exchangeris cooled by circulating it through an air to water heat exchangerthrough which air which is to support the combustion in the burner ofthe boiler passes on its route to the combustion chamber. In winter timethis air temperature typically can be in the range 0°–10° C. in the UK.

The second heat exchanger causes heat to be transferred to therelatively cold water therein due to the cooling of the flue gases aswell as by extracting latent heat from the water vapour content as thetemperature of the water vapour in the exhaust gases is reduced to belowits dew point.

Thus in recovering latent heat (which would otherwise be lost) all theexhaust gases are cooled to well below 100° C.

The products of combustion from a boiler are frequently at temperaturesin excess of 175° C., and therefore by cooling all the products ofcombustion, heat will be recovered from the gaseous components of theexhaust gases in the form of ordinary heat. This can vary from about 3%for a modern high efficiency boiler (where the exhaust gases may havealready been partially cooled) to as much as 8% or 10% for older lessefficient boilers, (from which the exhaust gas temperature willtypically be higher), and therefore will contain more heat energy to begiven up as these higher temperature gases are cooled to below the dewpoint of the water vapour present in the exhaust. Therefore applying aclosed loop water/hot gas (CLW/HG) exhaust gas cooling system to anolder inefficient. boiler, will provide an even greater efficiency gainthan when applied to a more efficient modern boiler.

It will be appreciated that the gain in efficiency is obtained byconsidering a building and a heating system as a whole, as distinct fromthe normal thinking which considers the boiler only when consideringefficiency.

If the water used to condense the water content of the exhaust by thesecond heat exchanger is to be re-circulated in a closed loop system,the water heated by the flue gases and the recovery of latent heat inthe second exchanger must itself be cooled before it can be reusedtherein, and the increase in overall efficiency is achieved bytransferring the energy so recovered either to the main boiler (todecrease the energy needed to heat the water flowing therethrough)and/or via a heat exchanger such as a radiator, to assist in heating theinterior of the building being heated by the boiler. This can bedirectly or via a third water to water heat exchanger

It should be noted that fuel oils, coal etc., can contain sulphur and onburning, the sulphur combines with oxygen to produce sulphur dioxidewhich will combine with water to produce sulphurous acid, which is acorrosive substance. Present in the atmosphere in aerosol form, afteremission from millions of domestic central heating boilers and thousandsof power stations, sulphurous acid returns to earth as so-called acidrain which is believed to cause damage to forests, lakes and possiblycauses or aggravates asthma and other allergic conditions in humanbeings, if not also other animal species.

In fact North Sea and LPG gases do not in the first instance containsignificant sulphur content. The sulphur compounds are in fact added bymanufacturers as “smellers” so that the presence of these gases can bedetected by smell when leaks occur, and it is these sulphur compounds inthe case of such gases which produce sulphur dioxide on burning.

If significant condensation of the water vapour content of the fluegases is achieved, sulphur dioxide (SO2) present in the flue gases willbe absorbed by the condensed water and, water being heavier than air,will collect at a low level and draining away this water automaticallydrains away the sulphurous acid.

If (as is proposed in preferred embodiments of the invention) there isdirect contact between cooling water and the flue Sulphur Dioxide (SO₂)therein will also be absorbed onto the cooling water, and will thereforenot be found in the exiting flue gases.

Where there is direct contact between cooling water and the flue gases,water condensing from the latter will mix with the cooling water andthereby become separated from the flue gases.

The invention also lies in the combination of a water boiler fuelled byburning a hydrocarbon fuel such as gas or oil and a building in whichthe boiler and radiators supplied with hot water from the boiler are alllocated, wherein heat which would normally be lost in the exhaust gasesleaving the boiler is recovered by a heat exchanger to which water atbelow the dew point of water vapour in the exhaust gases is supplied tocool the gases and recover the latent heat of vaporisation from thewater vapour, and wherein the recovered heat is employed to heat waterto be heated by the boiler and/or to heat a heat exchanger locatedwithin the building and/or to heat air entering the burner of theboiler, so that in terms of energy input to the building and energy lostfrom the building, the overall efficiency of the combination can beconsidered to be almost 100%.

In essence the boiler and building and flue can be considered as aclosed loop for recovering heat which would otherwise be lost in theflue gases. Such an installation can capture virtually all of the latentheat in the steam content of the exhaust gases, and by reducing thetemperature of the gases, will also extract additional heat from thegaseous content, thereby increasing the overall efficiency of theinstallation.

As a rule, not all rooms in a building are kept at the same temperaturein winter. Thus in a domestic dwelling, bedrooms, hallways and stairsfor instance are often kept somewhat cooler than living rooms. There aretherefore several locations in an ordinary domestic dwelling where heatwill be dissipated from a radiator or other heat exchanger quiteeffectively even if the temperature of the water supplied to it is equalto or less than the return flow temperature from the main radiatorsystem in the house. It is therefore quite feasible to locate a suitableradiator or heat exchanger in a building such as a domestic dwelling forthe purposes of the invention.

Thus according to the invention, a small amount of the water returningto the boiler from the radiator system of a building, possibly ½–1liter/minute, is diverted through an additional radiator or other heatexchange device in the building being heated, such that the exit watertemperature therefrom will be below the dew point temperature, and theexiting water is supplied to a secondary heat exchanger to which theexhaust gases from the boiler are also supplied to enable fullcondensing of water vapour in the flue gases from a boiler supplying hotwater to the radiators to be achieved.

Typically the exit water temperature is in the range 30°–40° C.

Preferably the secondary heat exchanger includes a first water/gasexchanger through which water returning from the radiators is circulatedto cool the exhaust gases and in which the water is kept separate fromthe exhaust gases, and by a second water/gas exchanger in which thegases and water may be in direct contact and in which the watertemperature is lower than that in the first heat exchanger.

Also according to the invention the water diverted through theadditional radiator or heat exchanger subsequently may be forced to flowthrough an air-cooled heat exchanger at the air inlet to the burner ofthe boiler before it is circulated through the said second heatexchanger, so as to cool the water to a temperature in the range 25°–30°C. by exposing it to cold incoming air, and in consequence heating theincoming air and thus increasing the burner efficiency.

The invention therefore also lies in a boiler (typically a fossil-fuelburning boiler) in which an air intake to the burner includes awater/air heat exchanger for heating the incoming air and in turncooling water circulating in the heat exchanger, to assist in condensingthe water content of the products of combustion of the boiler, beforethey exit to atmosphere.

A system embodying the invention thus allows the transfer of heat energyto the circulating water and/or to the air supply to the burner so as tobenefit the whole building heated by a boiler and hot water radiatorsystem by the full condensation of the flue gas water content from theboiler exhaust gas, collecting as it does the latent energy from thatwater content of the flue gases and recycling this energy back to thebuilding and/or to the boiler, therefore reducing fuel consumptionand/or increasing available energy for heating the building.

Where a building contains a reservoir of cold water in the roof spacefor supplying cold water to the building, the invention also envisagesthe provision of a water to water heat exchanger in that reservoir andmeans for circulating thereto some of the return flow water from theradiator system within the building either directly from the radiatorsor after passing through the said additional radiator or after passingthrough the air-cooled heat exchanger in the air inlet to the burner ofthe boiler, so as to cool water to be used to cool the flue gases whilesimultaneously warming the water stored in the high level reservoir,reducing energy needed to heat that water and also reducing thepossibility of the reservoir of water freezing in winter.

Preferably such an arrangement includes a thermostatically controlledvalve for restricting the flow of water to the said water to water heatexchanger if the temperature of the water in the storage reservoirexceeds a preset temperature.

In essence the invention provides a heating system for a buildingcomprising a boiler and radiators to which hot water is supplied fromthe boiler, in which at least some of the return water flow from theradiators typically at 60° C., circulates through a first heat exchangerto cool the exhausting products of combustion, means is provided todirect a small part of the return water flow through at least one heatexchanger which gives up heat to the building and/or the boiler heatingthe water for heating the building, to cool the diverted water to wellbelow the dew point, and means is provided to supply the cooled water toa second heat exchanger through which the already cooled exhaustproducts of combustion pass after leaving the first heat exchanger, toachieve full condensation of the water vapour content of the products ofcombustion from the boiler.

Furthermore the invention also provides a system which not only recoversthe latent energy in the water content of the exhaust gases but recyclesthis recovered energy back to the boiler thus reducing the boiler fuelconsumption.

The advantages of the invention may be seen by considering a domesticdwelling in the UK, in winter, when full boiler heat output will berequired. The house loses heat in a number of ways, notably throughwalls, roof, windows etc., but there are other causes of effective heatloss which are not normally taken into account.

One such is the effect of the temperature of the air drawn in to enablecombustion to occur in the boiler. In winter this temperature isconsidered to be 0°–5° C. in the UK. Most domestic boiler flue systemsare of the so-called balanced flue type. Essentially cold air fromoutdoors is drawn straight into the boiler burner system and it has beencalculated that in the case of an 80,000 BTHu boiler, the mass of thiscold air drawn in per hour is about 34 kg. The hourly aspiration of thismass of air at 0°–5° C. affects the boiler heat output since itrepresents a cooling effect in the combustion chamber and it has beencalculated that in such a boiler, the heat output is reduced byapproximately 0.5–0.75%.

Another loss of heat is what is wasted in the flue gases from the boilerheating the house. Thus a conventional non-condensing boiler exhaustsflue gases at a temperature which varies according to design but is mostlikely in the range 200°–250° C. These gases not only contain a quantityof what was once called “sensible heat”, (because it is possible tomeasure it directly) due to the high temperature of the gases, but alsocontains an appreciable quantity of latent energy stored in water vapourmixed with the other flue gases, due to the combustion of about 13.75%hydrogen in a typical fuel oil.

Another significant heat loss is directly due to the fact that a typicalhousehold consumes about 250 liters daily of water. In winter, thiswater is supplied to the house at a temperature of about 6°–8° C., andthis cold water has to be warmed or even boiled for many household uses.The energy required to do this in winter will therefore be higher thanin summer when the cold water (especially if stored in the house beforeuse) can be in the range 15°–20° C. Frequently the cold water is storedin a tank in a loft or attic, and unless carefully insulated such tankscan freeze in winter. The invention envisages a system which can help toavoid this problem.

It has been calculated that if a closed loop water/hot gas (CLW/HG)system is incorporated into a heating installation powered by a boileroperating at say 15 kw it will enhance the overall efficiency of theinstallation by typically from 9% to 15%. Put another way, the boilerfuel consumption will fall by a similar amount (i.e. typically from 9%to 15%), thus reducing CO₂ emissions by the same amount.

It can also be shown that if a CLW/HG system is coupled to a largerinstallation such as a hydrocarbon fuelled boiler in a power station,again significant improvement in overall efficiency can be obtained ifthe recovered heat energy is returned to the boiler for example to heatincoming air to the burner so as to increase its heat output or reducethe fuel needed to attain a particular burner temperature.

If a closed loop water/hot gas heating system proposed by the inventionwere to be adopted worldwide, CO₂ emissions would be reduced by up to9–15% annually, and where direct water/gas contact is provided for SO₂is also extracted therefrom, and one of the causes of acid rain would beremoved.

According to a preferred feature of the present invention, an advantageof a CLW/HG system embodying the invention and involving gas/watercontact is that both the efficiency and emissions of an existing boilercan be improved without the expense of fitting a new boiler, merely byfitting a closed loop water/hot gas exchanger to condense the steamcontent of the exhaust gases from the existing boiler, and recoveringand utilising the heat so gained therefrom.

The invention therefore also lies in a secondary heat exchanger adaptedto be fitted to the exhaust flue of an existing boiler and to beconnected to the water returning from the radiators heated by the boilerfor cooling the exhaust gases in the flue, and to be connected to afurther heat exchange means for cooling a second flow of watertherethrough to a temperature substantially below the dew point of thewater content of the gases in the flue.

A CLW/HG unit embodying the present invention can be constructed in anyconvenient manner and according to a preferred feature of the inventionsuch a unit is adapted to be accommodated into an existing boilerinstallation, in particular so as to fit inside the casing of such aboiler.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIGS. 1 and 2 are end and side elevation diagrammatic views of animproved secondary heat exchanger for use with a water heating boiler,which allows for direct water/flue gas contact;

FIGS. 3 and 4 illustrate alternative arrangements for cooling waterheated by direct contact with flue gases,

FIG. 5 is a side elevation diagrammatic view of another embodiment,providing improved direct flue gas/water contact and cooling heatedwater in a heat exchanger, and

FIG. 6 is a schematic diagram of a total system using an oil firedcondensing boiler which also removes sulphur dioxide from the fluegases.

CALCULATIONS OF SPECIFIC LATENT HEAT OF STEAM

According to published data, the specific latent heat of steam at 100°C.=2.26×10 kilojoules per kilogram or, more suitable for ourcalculations=2.26 megajoules per kilogram, (Mj/Kg)

Now in the case of a boiler consuming fuel at the rate of 1 USgallon/hour, this equates to 3.64 liters of fuel/hour.

The weight of fuel consumed per hour is obtained by multiplying thevolume by the density of the fuel. The density of one typical fuel is0.7945. Therefore the weight of such a fuel burnt (per hour)=2.892kilograms per hour.

If the gross fuel calorific value=46.08 Mj/Kg, then the total calorificvalue per hour is 2.892×46.08=133.2 Mj per hour.

If, as is typical, the percentage of hydrogen in the fuel=13.75%, thenthe weight of hydrogen in 2.892 Kg of fuel=2.892×0.1374=0.4 Kg, i.e. 0.4Kg of hydrogen is oxidised each hour.

Hydrogen reacts with 8 times its weight of oxygen to produce water. Thusthe weight of water produced per hour is 0.4×9=3.6 Kg of water per hour,albeit produced in the form of steam.

If the latent heat of steam=2.26 Mj/Kg, the latent heat of 3.6 Kg ofsteam is 3.6×2.26=8.136 Mj. This is the heat per hour contained in thesteam in the flue gases of such a boiler burning this “typical” fuel.

If the total energy of the burnt fuel is 133.2 Mj per hour, then thepercentage of the latent heat in the steam to the total energy is8.136/133.2=6% of the total energy from the fuel.

This figure should be seen in relation to the commonly held belief thatthe latent heat of steam in the boiler flue gases is approximately 3% ofthe total calorific value of the fuel burnt.

Also the total weight of steam per hour is 3.6 Kg, which is equivalentapproximately to 3.6 liters of water per hour, which by coincidence issimilar to the volume of the fossil fuel consumed. This is about 60cc/minute.

Modifications to the Secondary Heat Exchanger Proposed by UK PatentApplication No. 0107963.1

As has been described so far the previously proposed secondary heatexchanger extracts only a small percentage of the latent heat fromcondensing steam and the purpose of the present invention is tosubstantially increase the extraction rate of this latent heat.Reference is hereby made to the drawings and related description in UKPatent Application No. 0107963.1 for details of the design andfunctionality of the previously proposed secondary heat exchanger.

In the embodiment shown in FIGS. 1 and 2, direct flue gas cooling isachieved if condensate water is pumped from a reservoir 10 via a pump 12and pipe 14 to a perforated pipe 16 located above, centred and parallelto the upper face of a core 18, through which water at 60° C. or lessflows as it returns from a heating system to the main boiler heatexchanger. The design of core 18 may be such as is described for thesecondary heat exchanger in UK 0107963.1. Water from 16 will flow downthe external surfaces of the core as a film or sheet of water eventuallyto fall into and entirely fill by overflowing a shallow tray 20 whichmay comprise the reservoir 10, but is more preferably the top one of aplurality of similar trays 22, 24, 26 each of which overflows to feedthe one below and the lowest of which feeds the reservoir 10. The core18, perforated pipe 16, trays 20 to 26 and reservoir 10 are locatedwithin or formed by a housing 28.

In parallel the flue gases from the main boiler are fed into the housing28 via pipe 80 to mix with the water escaping from 16 and to traveltherewith as it passes over the core 18 and via trays 20 to 26 to thereservoir 10.

As the film of water cascades down the outside of the core, the hotgases will be cooled by contact therewith, and will continue to becooled as they traverse the surface of the water in the trays 20 et seq.In cooling, steam in the flue gases will be cooled below the steam/watertransition temperature and in doing so its latent heat will transferinto the water in the trays, and the condensing steam will also beabsorbed into the water. Thus simultaneously the water temperature andvolume will increase. The water flows into the reservoir 10 from whichit will be drawn by the pump 12 and recirculated via the perforated pipe16 to be cooled as it again makes contact with the surface of the core18 which is internally maintained at 60° C. by the return flow to themain boiler. As the process proceeds, the increase in volume of thecirculating condensate can be drawn off by overflow pipe 32, and thisexcess water could amount to several liters per hour depending on boilerheat output.

The path of the hot gases from inlet 30 to the final flue outlet 31 (seeFIG. 1) is denoted by arrows such as 33, 35.

If significant condensation is to be achieved the temperature of thecascading water needs to be below the dew point of the water vapour inthe flue gases. Typically this temperature should be below 40° C.,preferably below 30° C.

Second Embodiment

In the arrangement shown in FIGS. 3–5 the core is replaced by five waterfilled trays 34, 26, 38, 40, 42, each tray being individually cooled byone or more heat exchange tubes 44, 46, 48, 50, 52, arranged to extendacross each of the trays, and typically will be immersed by the water ineach tray when the system is operating. This design is more compact thanthat of FIGS. 1, 2 and has a larger flue gas cooling area containedwithin any given size of housing 28. Water overflowing from 52 iscollected in the well 54 at the bottom of the housing.

Hot gas entering the top of the housing from the pipe 30 encounters thewater surface of the first tray 34. This water is cooled as before bythe water returning to the main boiler. As the hot gases pass over andunder the trays, the gases are cooled first by contact with the watersurface and then with the underside of the tray.

It is possible that insufficient cooling of the gases will occur by tray34 to condense steam present in the gases into tray 34. In that event asmall pump (not shown) can be provided to circulate condensate from thewell 54 to tray 34, from which it will overflow and, by cascade, all thetrays will be constantly replenished.

The cooling tubes 44 et seq. of FIG. 5 may be of circular or square orrectangular cross section. Thus FIG. 3 shows a circular section pipe andFIG. 4 a rectangular cross section pipe. By careful selection of heightand width dimensions of the latter, an optimum pipe surface area can beachieved for any given size of tray, given that the larger theproportion of the tray cross section occupied by a pipe the smaller willbe the volume available for water in the tray.

An overflow pipe 32 is shown in FIG. 5 showing how if water in the well54 exceeds the height of the overflow pipe, water will leave 54 and canbe recovered for re-use elsewhere or simply drained to waste.

In a complete system, the “excess” water can be simply run to waste, orbe stored for use as low temperature hot water in a domestic, office orfactory environment, or allowed to cool naturally to external ambienttemperature for use as “cold” water, perhaps for irrigation or return towater reserves underground for subsequent recovery and use.

The passage of the hot gases from inlet pipe 30 to the final flue outlet56 is denoted by a series of arrows such as 58, 60.

As with the FIGS. 1, 2 embodiment, the temperature of the cascadingwater should be below the dew point of the water in the flue gases, andin practice should be below 40° C., and preferably below 30° C. toachieve a high level of condensation of the steam content of the fluegases.

Comparison of Cooling Capabilities

The total cooling surface of the original core 18 is typically 3500square centimeters.

Considering now the alternative arrangement of FIG. 5. If there are 5trays in the FIG. 5 embodiment, each 11 cms wide and 52 cms long, thenthe total surface area of the trays is 2860 sq. cms.

The exhaust gases also traverse the water in the well. If the width ofthe casing is greater than the width of the trays by 1 cm, the watersurface area in the well is 12×52=624 sq. cms.

Therefore the total gas/water interface is (2860+624)=3484 sq. cms.

The gas/metal interface is made up of the total surface area of thewalls of the trays. If each tray is 11 cms wide by 3 cms high, the wallarea per tray exposed to the passing exhaust gases is (3+3+11)×52=884sq. cms.

If there are 5 trays the total metal surface area will be 5×884=4420 sq.cms.

The total surface available for cooling the gases is given by adding theareas of the gas/water and gas/metal interfaces.

Therefore the total surface area for cooling is 3484+4420=7904 sq. cms.

The water in the trays is of course itself cooled by the return flow tothe boiler through the pipes, and if a small gap exists below each pipe(such as 44) the total external surface of each pipe will be in contactwith water. If the rectangular cross-section pipes have a cross-sectionof 7×2 cms, the surface area of the 5 pipes (each 52 cms long) will be5×(7+7+2+2)×52=4680 sq cms.

The area of 7904 sq. cms available in a FIG. 5 arrangement compares veryfavourably with the 3500 sq. cms of exhaust gas cooling surfaceavailable in the original core 18 of FIGS. 1 and 2. Furthermore thislarger area can be packed into a height of less than 30 cms, typically28 cms.

As with the earlier embodiments, the temperature of the water passingthrough the heat exchange tubes (or pipes) should be below the dew pointof the water vapour content of the flue gases, typically below 40° C.and preferably below 30° C.

Conventionally hot water leaving the boiler for the radiators is at 72°C. and typically the flow rate through the radiators is adjusted so thatthe return water temperature is of the order of 60° C. It is possible toarrange matters so that this return water is at a lower temperature suchas 30° C. or lower, so as to achieve useful condensation of thesteam/water vapour in the flue gases. This will of course raise thetemperature of the water before is passes to the boiler to be reheated,which is to advantage since it is normally considered desirable for thereturn water not to be too low in temperature.

Comparison with Other Fuels

The foregoing has assumed the fuel to be kerosene or the like. Howeversimilar advantages are obtained if the fuel is a gas such as naturalgas, Propane or Butane. These gases all contain hydrogen as follows:

-   -   Natural gas 23.18% by weight    -   Propane 17.98% by weight    -   Butane 17.21% by weight.

The recoverable latent heat from these fuels will be directlyproportional to the percentage of hydrogen present in the fuels.

Turning now to FIG. 6, the system shown will now be described in detail.

-   -   1. Commencing with the boiler 100, hot water at about 72° C. is        pumped around a number of heating devices, such as conventional        radiators 96. In the case of an ordinary heating system this hot        water is returned directly to the boiler for re-heating and        continuous recycling. As indicated above, the usual return        temperature of the water is about 60° C. The fall in temperature        of 12° C. relates to the heat given out by the radiators into        the building. In the arrangement shown in FIG. 6 the 60° C.        return water is directed via pipe 98 to the upper core 102 of a        secondary heat exchanger 104, in accordance with the invention.    -   2. At the same time flue gases from the boiler, typically at a        temperature in the range 150°–250° C., are led into the top of        the secondary heat exchanger via duct 106 and are free to flow        downward, over and around the external surface of the upper core        102. As this takes place the flue gases give up energy to the        core 102. The gas temperature falls to a temperature in the        range 75°–95° C., and the water flowing through core 102 rises        in temperature, typically by a few degrees centigrade. This        water, now typically at a temperature of 62° C. instead of 60°        C., is returned to the main boiler 100 via pipe 108. It should        be noted that this apparently small temperature rise should be        seen in relation to the 12° C. temperature drop across the        radiators mentioned above. This would give a probable boiler        efficiency of about 92% if no other steps were taken to improve        efficiency. In general there will be no condensation of water        vapour in the flue gases at temperatures of 75° C. or higher,        since dew point is most probably around 60° C.    -   3. Some of the 62° C. return water directed along the pipe 108        to return to the boiler 100 (typically about ½ liter per minute)        is directed into a household radiator 110. To achieve maximum        overall efficiency this radiator must be contained inside the        building which is heated by the other radiators 96 for reasons        which will be explained later. The radiator 110 should be of        such a size and be positioned in the building so that water        flowing therethrough at a rate of approximately ½ liter of water        per minute, and at a temperature of 62° C. on entry, will drop        to a lower temperature as it traverses and leaves the radiator.        Typically an exit temperature of 35°–40° C. is achieved but an        even lower temperature would be better still.    -   On leaving the radiator 110 the water, still flowing at about ½        liter per minute and at a temperature of say 35° C., is directed        via pipe 112 to a further heat exchanger 114 (typically a        socalled honeycomb heat exchanger) located in the air intake to        the burner of the boiler 100. The heat exchanger 114 is        therefore subjected to an inflow of air, the temperature of        which in the winter will typically lie in the range 0–10° C.,        typically 5° C., and on very cold days may even be lower that 0°        C.

The aim of heat exchanger 114 is to cool the water leaving the radiator110 to as low a temperature as possible, while still keeping this waterpart of the constant volume of water contained within the closed systemmade up of the boiler 100 and the radiators 96.

In the case of space heating systems, boilers such as 100 will normallyonly be operated in winter, and in the UK the external winter averageambient temperature is 5° C. (about 41° F.). It is at such times thatthe system should operate at the highest overall efficiency in terms offuel/energy conversion and with the least CO₂/SO₂ loss to atmosphere. Ofcourse, if the ambient temperature is much lower than 5° C., theprobability is that total efficiency of the system will be even higher.If the ambient temperature is higher, the amount of energy needed toheat the building will be less and therefore the quantity of fuel burntwill be less and the quantities of CO² and SO² will be proportionatelyless.

The need to obtain cold water is to enable water vapour in the fluegases to be condensed so as to recover the latent heat energy in that“low temperature steam”.

It can be shown that, in the case of a boiler operating at a ratedoutput of 80,000 BTHus per hour, the mass of air demanded by the burneris about 34 kilograms per hour. When this mass of air per hour is passedthrough the heat exchanger 114 at 5° C., water at 35° C. is found to becooled typically to 15–20° C., and the air entering the burner willitself be heated to a temperature typically in the range 20–30° C.

This cool water from 114 is now directed to the lower heat exchangercore 116 via pipe 118.

The lower part of 104 comprises a reservoir 120 for water 122 whichsurrounds and just covers the central core 116. The level is governed bya weir 124. Alternatively or in addition a syphon may be employed tomaintain the level.

The water 122 is cooled by the water in 116 to a temperature which willnormally be below the dew point of water contained in the flue gasesentering 104 via 106.

Water 122 from around 116 is drawn out by a pump 124 and delivered to amanifold 126 containing jet orifices providing a cascade of water formixing with descending flue gases. A tortuous path may be presented tothe water and gases by a plurality of horizontally spaced apart baffles128.

After passing down and around core 102, the flue gases will now be at atemperature in the range 75° C.–95° C. and these gases are mixed withthe water cascading from the manifold 126 and if provided around thebaffles in 128. The gases are thereby cooled before exiting to the flue130 and the area and number of the baffles 128 and volume of water flowfrom the manifold 126 are selected so as to achieve the desiredtemperature drop in the gases, prior to exit, so that the exittemperature is typically in the range 40° C.

In a domestic situation, the total area of the baffles is typically inthe range of 1–2 square metres, and since the water will cool both facesof the baffles the total cooling area will be twice that, i.e. typically2–4 square metres. Moreover, since the cooling of the gases is by directcontact with the cooled water, it is not difficult to ensure that thegas temperature falls to below the dew point of water vapour in the gasstream, so causing condensation and loss of heat from the vapour andsimultaneous cooling of the hot gases.

In general the baffles 128 should be packed as tightly as possible inthe space available so as to present a tortuous path for the exhaustgases and the water. In general the more baffles the better.

In the case of an 80,000 BTHu boiler, the expectation is that about 2liters per hour of water will condense out from the flue gases, and aflue gas temperature of 30° C.–40° C. should be achieved.

Calculations show that the energy recovered from the condensation ofwater vapour to produce 2 liters of water per hour, is 4.5 mega jouleswhich is equivalent to 1.3 kilowatts/hour. This energy will raise thetemperature of the water 122 circulating around core 116 unless it iscooled, and it is for this reason that it is necessary to remove eachhour slightly more than 1.3 kilowatts/hour from the ½ liter/minute waterflow. This energy of approximately 1.3 kilowatts/hour is equivalent toabout 4% of the energy provided by the radiators 96, where the waterflow, and the flow and return temperatures are as specified above.Therefore if radiator 110 (which dissipates this heat in order to coolthe return water to 35–40° C.) is located within the building which alsocontains the radiators, its heat output can be added to that from theradiators 96, so as to produce a theoretical efficiency of the order of96% for the system taken as a whole.

In addition however the transfer of heat to the incoming air to theboiler can increase the energy available from the boiler. For an 80,000BTHu boiler, this increase can be just over ½ kilowatt per hour. Thismeans that less fuel is needed to generate the rated heat output fromthe boiler, and a slightly increased overall efficiency obtained.Typically the further increase in efficiency can be of the order of ½%.

Total system energy efficiency can therefore be of the order of 96.5%.

The only loss is the overflow of water from 124.

Sulphur dioxide can be removed as follows:

From published information, sulphur dioxide is very soluble in water. AtNTP, a given volume of water will absorb 39 times that volume of sulphurdioxide. Thus if 30 liters of water per minute is cascading down thebaffle plates at 128 this volume of water will readily absorb far moresulphur dioxide than could possibly be present in the volume of exhaustgases passing through and present in the baffle containing region of theheat exchanger housing 104 in the same period of time.

As the water vapour condenses it will increase the volume of the water122 in the reservoir 120. This rising volume is controlled by weir 124.Water draining over the weir carries with it absorbed sulphur dioxidewhich can be neutralised by using the alkaline materials such as areused in domestic laundry facilities. Since the circulating water 122will also become acidic, it is of course necessary to remove thisacidity by a neutralising cartridge 132 which may be in the form of areplaceable or rechargeable unit. The pipework 134, pump 124, manifold126, baffles 128, core 116 and interior of the housing of the heatexchanger 104 must be formed from or coated with a material notchemically affected by sulphurous acid.

Whether spaced apart baffles 128 are employed, or the exchanger relieson mixing the gases with fine water sprays, one thing is common to allvariations, namely that the hot exhaust gases are cooled by directcontact with cold water, and the recovered sensible and latent heat ofcondensation thereby recovered from the flue gases, is incorporated intothe heat supplied to the building and/or to the boiler heating radiatorswithin the building so as to increase the overall efficiency of theinstallation.

1. A heating system for a building comprising a boiler and radiators forheating different regions of the building which are supplied with hotwater from the boiler and in which the boiler and radiators are all inthe building, wherein the boiler includes a burner for burning ahydrocarbon fuel such as gas or oil in a combustion chamber, and a heatrecovery system is provided by which heat which would normally be lostin the hot exhaust gases leaving the combustion chamber is recoveredtherefrom, wherein the heat recovery system comprises heat exchangemeans through which water returning from the radiators is caused toflow, and through which water at a temperature below the dew point ofthe water vapour in the flue gases is also caused to flow to cool thehot gases to recover the latent heat of vaporisation from the watervapour therein, and wherein the heat exchange means includes primary andsecondary water circuits, water in the secondary circuit being in directcontact with the hot flue gases so that the condensed water and anywater soluble products of combustion can mix with the cooling water andwill leave the heat exchange means with the cooling water, characterisedby means for diverting a proportion of the water returning to the boilerfrom the radiators from an exit in the return path to a further heatexchange means located within a region of the building which ismaintained at a lower temperature than other regions of the buildingthereby to cool the diverted water to a temperature below the dew point,means for conveying the cooled diverted water via the primary watercircuit in the heat exchange means to re-enter the return water path tothe boiler, downstream of the exit point from which it was diverted, toreturn to the boiler for reheating, and a pump is provided for pumpingwater in the secondary water circuit from a region in which it is cooledby the cooled diverted water flowing through the primary circuit to anupper region to form a cascade in which the hot gases are mixedtherewith.
 2. A heating system as claimed in claim 1 wherein the furtherheat exchange means includes a radiator in the lower temperature regionof the building.
 3. A heating system as claimed in claim 1 wherein thefurther heat exchange means includes a water to water heat exchanger ina reservoir of cold water provided for supplying cold water to thebuilding.
 4. A heating system as claimed in claim 3 which includes athermostatically controlled valve adapted to restrict the flow of waterto the said water to water heat exchanger if the temperature of thewater in the storage reservoir exceeds a preset temperature.
 5. Aheating system as claimed in claim 1 wherein the heat exchange meansincludes an air to water heat exchanger located in an air intake to theburner of the boiler through which air to support combustion passes onroute to the combustion chamber.
 6. A heating system as claimed in claim1 wherein the heat recovery system heat exchange means comprises firstand second heat exchange means through one of which water returning fromthe radiators is caused to flow, and through the other of which water ata temperature below the dew point of the water vapour in the flue gasesis caused to flow.
 7. A system as claimed in claim 1 wherein the volumeof diverted water is in the range of 0.5–1.0 liters/minute.
 8. A methodof cooling the products of combustion (flue gases) of a fossil (i.e.hydrocarbon based) fuel burning water heating apparatus (a boiler) whichin use heats water for circulation by a pump around a closed circuitcontaining radiators and a main heat exchanger by which it is heated bythe burning of the fossil fuel in air in a burner in the boiler, andwherein the flue gases pass through secondary heat exchange means bywhich they are cooled below the dew point temperature of water vapour inthe flue gases so that the water condenses and is thereby separated fromthe flue gases, and wherein a portion of the circulating water isdiverted from the main flow at a point just prior to its return to themain heat exchanger, and the diverted flow is cooled by further heatexchange means to a temperature substantially below the dew pointtemperature, and wherein the cooled water is employed in the condensingheat exchange step in the secondary heat exchange means to cool thegases below the said dew point and is thereafter returned to the mainreturn water flow to the main heat exchanger downstream of the point atwhich the portion of the circulating water was diverted.
 9. A method asclaimed in claim 8 wherein the said condensing cooling step involves amixing of the flue gases and the cooled circulating water, and thecondensing of the water vapour by the said further cooling stepincreases the volume of water circulating around the generally closedcircuit and an overflow or syphon is provided to maintain the volume ofcirculating water substantially constant, the water and flue gases areseparated using gravity before the flue gases exit to atmosphere, andthe recovered water drains into a low level reservoir forming part ofthe said generally closed circuit and a coiled pipe in the reservoirprovides a path for the low temperature water by which the heat isexchanged between the water in the reservoir and the low temperaturewater, to cool the water in the reservoir to below the dew point.
 10. Amethod as claimed in claim 8 wherein the flue gases are progressivelycooled in the secondary heat exchange means by a said first cooling stepto a temperature above the dew point, and thereafter by the condensingcooling step in which they are cooled to below the dew point by lowtemperature water which is circulated around a generally closed circuitand which is cooled to substantially below the dew point temperature.11. A method as claimed in claim 10 wherein the said condensing coolingstep involves a mixing of the flue gases and the cooled circulatingwater, and the condensing of the water vapour by the said furthercooling step increases the volume of water circulating around thegenerally closed circuit and an overflow or syphon is provided tomaintain the volume of circulating water substantially constant, thewater and flue gases are separated using gravity before the flue gasesexit to atmosphere, and the recovered water drains into a low levelreservoir forming part of the said generally closed circuit and a coiledpipe in the reservoir provides a path for the low temperature water bywhich the heat is exchanged between the water in the reservoir and thelow temperature water, to cool the water in the reservoir to below thedew point.